The present invention relates to a steam governing valve used in a steam turbine, and in particular, to a steam governing valve in which the vibration of the valve body in a large opening degree of the valve body can be suppressed and a stabilized motion of the valve body can be obtained.
As shown in FIG. 6 a conventional steam governing valve comprises a casing body 1, an upper cover 2, a bush 3 secured to the upper cover 2, a valve rod 4 slidably fitted in the bush 3, a sleeve 7 secured to the casing body 1, a valve body 6 slidably fitted in the sleeve 7 with a clearance 8 interposed therebetween and forming a main valve 15, control means for controlling the valve rod 4, a smaller valve body 13 formed at the lower end portion of the valve rod 4, a smaller valve seat 14 formed on the valve body 6, a main valve seat 16 formed on the casing body 1, an entrance chamber 18 formed in the casing body 1, an inlet port 21, an outlet port 22, a pressure chamber 19 defined by the valve body 6, the sleeve 7 and the upper cover 2, and a flow passage 20 formed in the valve body 6.
The valve rod 4 is formed at its lower portion with a valve rod shoulder 5, and the valve body 6 is formed with a projection 6a adapted to engage with the shoulder 5.
The control means for controlling the valve rod 4 are composed of a hydraulic actuator 9, a lever 11 connected at one end thereof with a piston rod 10 of the hydraulic actuator 9 and at the other end thereof with the valve rod 4 through a pin 17 and rotatably supported at its middle portion by a stationary member, and a compression spring 12 always pressing the valve rod 4 downwards with a constant force Fs.
A vertical motion of the piston rod 10, produced by increasing or decreasing the hydraulic pressure of the hydraulic actuator 9, is transmitted to the valve rod 4 through the lever 11. When the valve rod is moved upwardly, the compression spring 12 is compressed. When the lift amount of the valve rod 4 is smaller than the maximum lift amount l, the valve body 6 continues to be in close contact with the main valve seat 16, and the valve rod shoulder 5 is separated from the projection 6a. As a result, a high pressure steam flows from the entrance chamber 18 through the clearance 8 between the sleeve 7 and the valve body 6 into the pressure chamber 19, and then flows out through a gap between the valve rod shoulder 5 and the projection 6a and a gap between the smaller valve body 13 and the smaller valve seat 14, and through the flow passage 20 towards the outlet port 22.
When the valve rod 4 is further lifted upwardly beyond the maximum lift amount ( of the smaller valve, the valve rod shoulder portion 5 engages with the projection 6a and moves the valve body 6 upwardly. As a result, the valve body 6 is separated from the main valve seat 16, and the main valve 15 is opened to a opening degree L. In this state, the steam in the entrance chamber 18 flows out directly to the outlet port 22.
FIG. 7 shows the pressure P.sub.1 at the entrance chamber 18, the pressure P.sub.3 at the pressure chamber 19 and the pressure P.sub.2 at the outlet port 22 in relation to the valve opening degree in the above-mentioned state of the valve.
When the valve body 6 is moved to a position corresponding to a predetermined valve opening degree L, the valve rod 4 receives a force F.sub.P which depends on the pressure P.sub.1 in the entrance chamber 18, the pressure P.sub.3 in the pressure chamber 19 and the pressure P.sub.2 at the outlet port 22. The relationship between the above-mentioned force F.sub.P, the force F acting on the valve rod 4 from the hydraulic actuator 9 and the force F.sub.S from the compression spring 12 is expressed by the following equation: EQU F=F.sub.P +F.sub.S
The compression force F.sub.S of the compression spring 12 linearly varies according to the change of the valve opening degree, while the force F.sub.P depends upon various pressures of the steam varies in a complex manner according to the valve opening degree. Considering the force F.sub.P acting on the valve rod 4, as shown in FIG. 8, the force F.sub.P is very small until the smaller valve body 13 is fully opened, sharply increases the instant when the main valve 15 starts to open, and gradually decreases as the valve opening degree of the main valve increases. This force F.sub.P is expressed in the following equation (1): ##EQU1## where, P.sub.1 ; pressure in the entrance chamber 18,
P.sub.2 ; pressure at the outlet port 22, PA0 P.sub.3 ; pressure at the pressure room 19, PA0 P.sub.a ; atmospheric pressure, PA0 D.sub.1 ; outside diameter of the lower end of the valve body 6 PA0 D.sub.2 ; diameter of the outlet port 22, PA0 D.sub.3 ; diameter of the flow passage 20, and PA0 D.sub.4 ; diameter of the valve rod 4.
In FIG. 8, in a range of the valve opening degree from zero to l, the term (c) is predominant in the equation (1), and the force F.sub.P is negative, because P.sub.a &lt;P.sub.3.
When the valve rod shoulder 5 engages with the projection 6a and the main valve 15 starts to open, the terms (a), (b) and (c) are all effective in the equation (1), and the force F.sub.P acting on the valve rod 4 starts to sharply increase. When the valve opening degree further increases, the pressure P.sub.2 increases as shown in FIG. 7. As a result, the pressure differences (P.sub.1 -P.sub.2) in the term (a) of the equation (1) and (P.sub.3 -P.sub.2) in the term (b) decrease, and accordingly, the force F.sub.P acting on the valve rod 4 decreases. At a valve opening degree nears the full opening L.sub.o, the force F.sub.P becomes very small, namely regarded as F.sub.P .apprxeq.0. In other words, the engaging force for maintaining an engagement between the valve rod 4 and the valve body 6 becomes very small, and the motion of the valve body 6 becomes very unstable in the valve axial direction (direction along the force F.sub.P), although it is guided by the sleeve 7.
In a steam governing valve used near at the full opening degree L.sub.o, the steam flows at high speed through the main valve 15, and diffuses to the outlet port 22. As a result, in the downstream side of the main valve 15, a steam flow state including severe disturbances is generated. The pressure variation caused by the above-mentioned flow disturbances acts on the valve body 6, thereby producing an abnormal vibration of the valve body 6 which is in an unstable condition as mentioned above.
The characteristic of the force F.sub.P acting on the valve rod 4 of the prior art shown in FIG. 8 is a result of providing the smaller valve 13. The advantage of providing the smaller valve 13, as understood from FIGS. 7 and 8, is that the force acting on the valve rod 4 when the main valve starts to open can be decreased by .DELTA.F, and accordingly, a hydraulic actuator having a small size and a light weight can be obtained. However, there is a disadvantage that the engaging force between the valve rod 4 and the valve body 6 is small when the main valve is largely opened as mentioned above, and the valve body 6 is brought into an unstable condition.
Japanese Patent Laid-open No. 62-147002 discloses a steam governing valve improved with respect to the steam flow state around the valve body when the main valve 15 is slightly opened. In this steam governing valve, each of the sleeve 7 and valve body 6 is formed with through holes extending in directions perpendicular to the valve axis, and these through holes overlap each other when the smaller valve 13 has been fully opened and the main valve 15 is slightly opened, thereby producing a communication between the entrance chamber 18 and the pressure room 19. By virtue of this arrangement, the steam flow rate through the flow passage 20 of the smaller valve 13 is increased, the mixing of the steam flow through the flow passage 20 with the steam flow from the main valve 15 is improved, and the steam flow at the outlet port 22 is stabilized, thereby decreasing the force of the fluid which may excite the vibration of the valve body 6. However, as mentioned above, the overlapping or communication between the through holes is obtained only in a range where the main valve opening degree is small, but no communication is obtained in a range where the main valve opening degree is near the fully open state. Therefore, the steam governing valve of this type has also a disadvantage that the motion of the valve body become unstable when the valve is moved to near the full open position.
The object of the present invention is to provide a steam governing valve in which the vibration of the valve body is suppressed even when the valve body moves in the valve opening direction beyond a predetermined valve opening degree, and the motion of the valve body is stabilized.